The present invention relates generally to microalloy steels and, more particularly, to microalloy steels that can be forged and subsequently surface hardened by induction heating. Microalloy forging steels have been finding increasing usage in many applications, but perhaps the fastest growing domestic microalloy application is that of gasoline and diesel engine crankshafts. Most new crankshaft applications specify microalloy steel, and many current applications are being converted from cast iron or forged and heat treated plain carbon or alloy steels to as-forged microalloy steels. The engines range from small automotive to large diesel engines. Induction hardening is applied mainly in the larger diesel engine size ranges to enhance fatigue strength of the bearing and crankpin fillets. It has been previously shown that a microalloy steel could uniformly attain the necessary core properties for large diesel engine crankshafts in the as-forged condition. However, until recently, the induction hardening characteristics of the as-forged microalloy crankshaft and resultant fatigue life had not been fully studied.
Induction hardening is a selective hardening process that has been traditionally applied to plain carbon and alloy steel components to increase the local hardness in highly stressed regions of the part. During induction hardening, the surface region of the part adjacent to the induction coil is rapidly heated to within the austenite regime, held at temperature for a brief period of time, and then rapidly quenched. The goal is to fully austenitize the heated region to a specified depth, and then form a martensitic structure in the heated region during the quench. The part is then tempered to the desired case hardness. The final strength/hardness of this induction hardened region has been the primary design criteria used to predict or establish the life of the part. Typically, the fatigue strength is considered to be approximately 1/2 that of the tensile strength of the hardened region. Whereas this general rule gives a good approximation of the performance characteristics of the part, optimizing the fatigue strength of an induction hardened component requires an understanding of how a number of metallurgical factors influence fatigue. In addition to the strength of the hardened region, the cleanliness of the steel and the resultant residual stress state in the highly stressed region will affect fatigue performance. Additionally, an understanding of how the induction hardening process affects the microalloy precipitate distribution in the base material is of importance.
With the progression towards cleaner steels in recent years, the effect of material cleanliness on bending fatigue is tending to play a smaller overall role with respect to fatigue properties. Whereas the oxide level has been dramatically decreased, the use of some level of sulfur to aid in machinability is still widely accepted in the art for many applications. Crankshafts in particular are subjected to a variety of forging and subsequent machining operations. Therefore, the use of sulfur is critical to allow the productive drilling of various holes required in a finished crankshaft. The presence of sulfides in steels has been known to play an insignificant role in regard to fatigue due to the lack of significant stress field around this type of inclusion. Heretofore, it has been shown that sulfides can actually improve fatigue resistance when they act to displace or coat more harmful oxide-type inclusions. However, in very clean steels, sulfides have been reported to initiate fatigue, and decreasing sulfur level has been shown to improve the endurance limit in both carburized and through-hardened steels. It is, therefore, of benefit to characterize the role of sulfur in the ultra clean air melt steels to find the proper balance between machinability and fatigue strength in critical applications.
Induction hardening to increase the hardness in fatigue critical regions of components also influences the resultant residual stress state. The expansion that accompanies the austenite to martensite transformation in the induction heated surface region normally results in a highly compressive residual stress at the hardened surface. This compressive stress, in combination with the increased hardness level, further enhances the fatigue strength of the component. Since a tensile stress is required to initiate a fatigue crack, the applied stress must overcome both the residual compressive stress present at the surface and the inherent strength of the steel to initiate a crack. One prior art investigator, Koster, "Surface Integrity of Machined Materials", Technical Report AFML-TR-60 (1974), working with SAE 4340 steels hardened to 50 HRC over a wide range of surface residual stress states, reported a near linear relationship between residual stress level and bending fatigue strength. It is also reported in the prior art that relaxation of harmful tensile residual stress by annealing will improve the fatigue strength for the finished part.